MORPHOLOGY OF ANIMALS 



Fig. S- Diagrammatic Transverse Sections of the bodies of A, Coelenterate ; B, Flatworm ; 

 C, Annelid ; D, Vertebrate ; a, blastocoel filled with mesenchyme cells {m) ; G, Gastrocoel 

 lined with endodermal epithelium ; g, gonocoel or cavity of sex ducts and glands, probably 

 homologous with c, coelom of animals with true body cavity ; n, nerve tube ; nc, notochord. 

 The walls of the coelom are composed of a thin layer of mesothelium (peritoneum), a thick 

 muscular layer (solid black), a thin layer of mesenchyme cells (ot) in what was earlier the 

 blastocoel {a), and covering the outside of the body the ectodermal epithelium, while endo- 

 dermal epithelium lines the gastrocoel (G) ; d, dorsal mesentery ; v, ventral mesentery, which ^ 

 has almost disappeared in the vertebrate (D). 



In the higher segmented animals the various somites 

 are no longer alike (homonomous), but show physiological 

 division of labor, some being differentiated for one func- 

 tion and some for another (heteronomous). In this way 

 some of the organs named above disappear in certain seg- 

 ments, while others become greatly enlarged or modified. 



C 21 3 



